We quantitatively investigate the size-dependent optical properties of colloidal PbS nanocrystals or quantum dots (Qdots) by combining-the Qdot absorbance spectra with detailed elemental analysis of the Qdot suspensions. At high energies, the molar extinction coefficient epsilon increases With the Not volume d(3) and agrees with theoretical calculations using the Maxwell-Garnett effective medium theory and bulk values for the Qdot dielectric function. This demonstrates that quantum confinement has no influence on E in this spectral range, and it provides an accurate method to calculate the Qdot concentration. Around the band gap, epsilon only increases with d(1.3), and values are comparable to the epsilon of PbSe Qdots. The data are related to the oscillator strength f(if) of the band gap transition and results agree well with theoretical tight-binding calculations, predicting a linear dependence of f(if) on d. For both PbS and PbSe Qdots, the exciton lifetime tau is calculated from f(if). We find values ranging between 1 and 3 mu s, in agreement with experimental literature data from time-resolved luminescence spectroscopy. Our results provide a thorough general framework to calculate and understand the optical properties of suspended colloidal quantum dots. Most importantly, it highlights the significance of the local field factor in these systems.

@article{780823,
abstract = {We quantitatively investigate the size-dependent optical properties of colloidal PbS nanocrystals or quantum dots (Qdots) by combining-the Qdot absorbance spectra with detailed elemental analysis of the Qdot suspensions. At high energies, the molar extinction coefficient epsilon increases With the Not volume d(3) and agrees with theoretical calculations using the Maxwell-Garnett effective medium theory and bulk values for the Qdot dielectric function. This demonstrates that quantum confinement has no influence on E in this spectral range, and it provides an accurate method to calculate the Qdot concentration. Around the band gap, epsilon only increases with d(1.3), and values are comparable to the epsilon of PbSe Qdots. The data are related to the oscillator strength f(if) of the band gap transition and results agree well with theoretical tight-binding calculations, predicting a linear dependence of f(if) on d. For both PbS and PbSe Qdots, the exciton lifetime tau is calculated from f(if). We find values ranging between 1 and 3 mu s, in agreement with experimental literature data from time-resolved luminescence spectroscopy. Our results provide a thorough general framework to calculate and understand the optical properties of suspended colloidal quantum dots. Most importantly, it highlights the significance of the local field factor in these systems.},
author = {Moreels, Iwan and LAMBERT, KRIS and Smeets, Dries and De Muynck, David and Nollet, Tom and Martins, Jos{\'e} and Vanhaecke, Frank and Vantomme, Andre and Delerue, Christophe and Allan, Guy and Hens, Zeger},
issn = {1936-0851},
journal = {ACS NANO},
keyword = {MICROCRYSTALS,SEMICONDUCTOR NANOCRYSTALS,ELECTRONIC-STRUCTURE,CDSE,DIELECTRIC CONFINEMENT,EXTINCTION COEFFICIENT,ROUTE,ABSORPTION CROSS-SECTION,GLASSES,SPECTROSCOPY},
language = {eng},
number = {10},
pages = {3023--3030},
publisher = {AMER CHEMICAL SOC},
title = {Size-Dependent Optical Properties of Colloidal PbS Quantum Dots},
url = {http://dx.doi.org/10.1021/nn900863a},
volume = {3},
year = {2009},
}